Photolithography is a well known process for transferring geometric shapes present on a mask onto the surface of a silicon wafer. In the field of IC lithographic processing a photosensitive polymer film called photoresist is normally applied to a silicon substrate wafer and then allowed to dry. An exposure tool is utilized to expose the wafer with the proper geometrical patterns through a mask by means of a source of light or radiation. After exposure, the wafer is treated to develop the mask images transferred to the photosensitive material. These masking patterns are then used to create the device features of the circuit.
One important limiting characteristic of any exposure tool is its resolution limit. The resolution limit for an exposure tool is defined as the minimum feature that the exposure tool can repeatedly expose onto the wafer. Currently, the resolution limit for most advanced optical exposure tools is around 0.4 micron, i.e. close to the smallest dimension (referred to as the critical dimension or CD) for many current IC layout designs. As a result, the resolution of the exposure tool may influence the final size and density of the IC circuit.
Another important characteristic of an exposure tool is its depth of focus, (DOF). The DOF of an exposure tool is defined as the range in which the aerial image (of a near resolution sized feature) will stay in focus. In a lithographical process in which an image is transferred into a resist layer, a minimum DOF is required. This minimum DOF ensures that the image remains sufficiently in focus throughout the resist layer. Thus, the minimum DOF range is typically greater than or equal to the thickness of the resist layer.
The DOF of an exposure tool determines the "usable resolution" setting of the exposure tool. For instance, if an exposure tool is able to resolve 0.4 micron features but has a DOF range less than the range needed to clearly focus this feature through-out the resist layer, then the 0.4 micron resolution setting can not be utilized. As can be seen, if the range of DOF of an exposure tool can be extended, the "usable" resolution limit may be decreased and smaller images may be printed.
A simplified diagram of a conventional exposure tool is shown in FIG. 1. As can be seen light source 200 projects light waves 208 through opening 202 in aperture stop 201. Opening 202 is commonly referred to as the pupil of the aperture stop. Condenser lense 205 collects the light from pupil 202 and focuses it on mask 206 such that the mask is evenly illuminated. When illuminating beam 203 passes through mask 206, imaging beam 209 is generated. Imaging beam 209 is projected through lens 207 such that the image of the pattern on the mask is focused onto the silicon wafer.
As can be seen in FIG. 1, pupil 202 is situated in the center of aperture stop 201. Because of this, illuminating beam 203 is projected along the optical axis (dashed line 204) from pupil 202 to condenser lense 205 and mask 206. This type of illumination method is called "On-axis illumination"-the name implying that the illumination beam is "on" the optical axis. FIG. 3 shows a planer view of an on-axis illuminator aperture stop 201. As is seen in FIG. 3, an on-axis aperture stop characteristically has the pupil opening disposed in the center.
In a attempt to reduce device sizes, the semiconductor industry is currently investigating a new way to reduce the "usable" resolution of the exposure tool by extending its associated DOF range. Recently, it has been noted that the DOF range of an exposure tool can be extended by changing the manner in which the mask pattern is illuminated. Specifically, it has been found that by projecting the illuminating beam at an angle other than that of the optical axis, the DOF of an exposure tool may be extended. This type of illumination technique is referred to as off-axis illumination.
FIG. 2 illustrates a simplified diagram of an exposure tool that provides off-axis illumination. Light source 200 projects light waves 208 to aperture stop 201'. As can be seen, unlike aperture stop 201 (FIG. 1), aperture stop 201' has two off-centered pupil openings. The modified aperture stop causes illuminating beam 203 to be projected from condenser lense 205 to mask 206 at an angle other than optical axis 204.
FIGS. 4 and 5 show planer views of two preferred types of off-axis aperture stops. FIG. 4 shows an aperture stop that provides one type of quadrapole illumination and the aperture stop shown in FIG. 5 provides an annular type of illumination.
As the size of integrated circuit layout designs are reduced, the critical dimension of the layout design frequently approaches the resolution limit of the exposure tool. In both on-axis and off-axis illumination exposure tools, when this occurs, inconsistencies between masked and actual layout patterns developed in the photoresist become significant. These inconsistencies occur due to various affects.
One primary affect that has drawn significant attention in the field of lithography is referred to as proximity effects. Proximity effects occur when adjacent features interact with one another in such a way to produce pattern-dependent variations. For example, lines designed to have the same dimension, but which are placed in different proximity to other features in a layout (isolated vs. densely packed), will not have the same dimensions after being developed. Thus, a group of densely packed lines tends to transfer differently when compared with an isolated line. Obviously, significant problems can arise in an IC when line widths are not consistently reproduced.
Another prevalent problem that occurs when CDs approach the resolution limit of an exposure tool is seen when printing a mask having both square and rectangular contacts. This type of mask characteristically has many variably sized, openings surrounded by large opaque areas. Inconsistencies in CDs between large and small contact openings developed into the resist layer occurs because each different sized contact has a different exposure energy requirement. In other words, the optimal energy setting for a large contact is much less than the optimal energy setting for a smaller contact. However, since only one energy setting can be utilized to expose a single mask, only one contact type is optimally transferred. The other contact type will be either over or under exposed.
Numerous solutions are available to compensate for both proximity effect problems and inconsistencies in contact CDs. One solution to the proximity effect problem is described in U.S. Pat. No. 5,242,770 assigned to the assignee of the present invention. This patent describes an improved mask comprising additional unresolvable lines that adjust the edge intensity gradient of isolated edges in the mask pattern. The isolated edge gradients are adjusted to match the, edge intensity gradients for densely packed edges. As a result, isolated and densely packed features transfer similarly and proximity effects are significantly reduced.
Further, a solution to reduce CD inconsistencies in contacts is disclosed in U.S. Pat. No. 5,256,505 assigned to the assignee of the present invention. In U.S. Pat. No. 5,256,505, energy levels of large and small contacts are matched by adding lines of opposite transparency within the larger features in the mask pattern to dim their energy intensity levels. As a result, energy requirements are matched for both large and small contacts and both types of features transfer within acceptable CD range of one another.
Although U.S. Pat. No. 5,242,770 solves proximity problems in on-axis illumination exposure tools, it is not fully effective in reducing these problems when off-axis illumination is utilized. One reason for this is because off-axis illumination significantly increases the DOF range of densely packed features but provides little DOF improvement for isolated lines. As a result, in some ways off-axis illumination has made the proximity problem even tougher to manage due to this DOF difference.
Similarly, although U.S. Pat. No. 5,256,505 provides a way of printing large and small contacts in a single mask having consistent CDs, it does not exploit the increased DOF ranges offered by off-axis illumination in larger contacts.
In conclusion, although off-axis illumination has its drawbacks, it is generally agreed in the semiconductor industry that this type of illumination can effectively be applied to extend "usable" resolution by extending DOF ranges, if the proximity and DOF differences between isolated and packed features are minimized and contact transference problems are addressed. What is needed is a means to take advantage of the increased DOF offered by off-axis illumination while also providing a means to handle proximity and contact problems.